1. Field of the Invention
This invention relates to a method of electronically attenuating a polar grid based on the torque profile of a positive displacement pump in order produce a constant pump pressure regardless of pump radial crankshaft/camshaft/crankarm location and the velocity of the fluid being pumped. In the method, an electronic processor compares the shaft displacement angle of the pump input shaft to a reference polar grid of torque profile and varies the electrical power applied to the pump motor. The processor can also take into account the response time of the pump drive, the motor inductive reactance, system inertia, application characteristics of the pump, and regenerative energy during deceleration of the pump.
2. Description of the Prior Art
In the prior art, it is well known that in situations where higher pressures of fluid movement are desired, a positive displacement pump is commonly used. A positive displacement pump is usually a variation of a reciprocating piston and a cylinder, of which the flow is controlled by some sort of valving. Reciprocal machinery, however can be less attractive in use than rotary machinery because the output of a reciprocal machine is cyclic, where the cylinder alternatively pumps or fills, therefore there are breaks in output. This disadvantage can be overcome to a certain extent by: using multiple cylinders; bypassing the pump output through flow accumulators, attenuators, dampers; or waste gating the excess pressure thereby removing the high pressure output of the flow.
In addition to uneven pressure and flow output, reciprocating pumps have the disadvantage of uneven power input proportional to their output. This causes excessive wear and tear on the apparatus, and is inefficient because the pump drive must be sized for the high torque required when the position of the pump connecting rod is at an angular displacement versus crankarm dimension during the compression stroke that would result in the highest required input shaft torque.
Moreover, if the demand of the application varies, complicated bypass, recirculation, or waste gate systems must be used to keep the system from xe2x80x9cdead-headingxe2x80x9d. That is, if flow output is blocked when the pump is in operation, the pump will either breakdown by the increased pressure or stall. If stalling occurs, a conventional induction electric motor will burn out as it assimilates a locked rotor condition with full rated voltage and amperage applied. Typically systems with fixed displacement pumps use a relief valve to control the maximum system pressure when under load. Therefore, the pump delivers full flow at full pressure regardless of the application thus wasting a large amount of power.
In this regard, certain prior art that attempts to correct the problems associated with torque output of a pump motor should be noted.
In U.S. Pat. No. 5,971,721, an eccentric transmission transmits a torque demand from a reciprocating pump, which varies with time, to the drive motor such that the torque demand on the drive motor is substantially constant. The result is the leveling of torque variation required to drive a positive displacement pump at the transmission input shaft with the effect of constant pump output pressure. This is accomplished by means of eccentric pitch circle socket sets with gear belts or eccentric pitch circle matched gear sets.
The use of an eccentric gear or sprocket set, has a significant effect on the overall torque requirement and the magnitude of the discharge pulsation of the pump but because most pumps are of a multi-cylinder or are vane or gear types, the pump input shaft torque requirement would not be perfectly counter-acted (leveled) by using the reduction pattern developed by eccentrically matched transmission components.
In U.S. Pat. No. 5,947,693, a position sensor outputs a signal by sensing the position of a piston in a linear compressor. A controller receives the position signal and sends a control signal to control directional motion output from a linear motor.
In U.S. Pat. No. 4,726,738, eighteen or nineteen torque leads are measured during main shaft in order to maintain constant shaft velocity. revolution and are translated to a required motor torque for particular angles of the main shaft.
U.S. Pat. No. 4,971,522 uses a cyclic lead transducer input and tachometer signal input to a controller to signal varied cyclic motor input controls to provide the required motor torque output. A flywheel is coupled to the motor in order to maintain shaft velocity. However, the speed of the motor is widely varied and the torque is varied to a smaller extent. U.S. Pat. No. 5,141,402 discloses an electrical current and frequency applied to the motor which are varied according to fluid pressure and flow signals from the pump. U.S. Pat. No. 5,295,737 discloses a motor output which is varied by a current regulator according to a predetermined cyclic pressure output requirement. The motor speed is set to be proportional to the volume consumed and inversely proportional to the pressure.
It is seen from the foregoing that there is a need for electronic attenuation of torque profile in a pump. When torque profile is compared with input shaft displacement and other factors such as system inertia, and response time of the pump drive etc., a pump can produce constant pressure at the full range of the designed system volume.
It is therefore an object of the present invention to provide a method for electronic attenuation of pump torque variation requirements in order to produce a matched motor torque output that will result in constant output pressure from a pump.
It is therefore a still further object of the present invention to provide control factors which vary the power and torque output of a pump motor based on calculated torque variation requirements.
It is therefore a still further object of the present invention to increase the energy efficiency of a pump by providing a force balanced relationship between the motor output and application hydraulic requirement.
It is therefore a still further object of the present invention to decrease the wear and tear on the pump by providing a substantially constant force output from the motor of the pump and reduce the amount of cycles of the pump to the application requirement.
It is therefore a further object of the present invention to provide a method for electronic attenuation of pump torque variation supplying information for design of a transmission system that can achieve a constant force output from the motor to the pump.
To attain the objects described, there is provided a method for obtaining a polar map for process control within the electronic drive of a targeted pump. This polar map is calculated by a processor or externally calculated then input into a processor. Once the torque profile of the pump is obtained and translated into a polar map, the processor can compare the shaft displacement angle of the pump input shaft to the reference polar map. The processor can also take into account selected factors such as the response time of the pump drive, the motor inductive reactance, system inertia, application characteristics of the pump, and regenerative energy during deceleration of the pump. Using selected factors and the comparison results the processor then signals the motor controller to vary the amperage, voltage, and frequency applied to the motor in order to regulate the torque output of the pump motor. With an accurately regulated motor power output, the pump output pressure will remain constant regardless of pump crank arm location or the velocity of fluid flow.